P626: FUNCTIONAL SCREENING OF PI3K INHIBITORS STRATIFIES RESPONDERS TO IDELALISIB AND INDICATES DRUG CLASS ACTIVITY IN IDELALISIB-REFRACTORY CLL
Authors: S. Skånland, Y. Yanping, P. Athanasiadis, L. Karlsen, A. Urban, I. Murali, S. Fernandes, A. Hilli, K. Taskén, F. Bertoni, A. Mato, E. Normant, J. Brown, G. Tjønnfjord, T. Aittokallio
Journal: HemaSphere (2022)

Background: The phosphatidylinositol 3-kinase inhibitors (PI3Ki) idelalisib and duvelisib are approved for relapsed chronic lymphocytic leukemia (CLL). While patients may show an initial response to these therapies, development of treatment resistance or intolerance remains clinical challenges. Prediction of individual treatment responses based on clinically actionable biomarkers is needed to overcome these challenges. Aims: 1. To characterize functional responses to 10 PI3Ki in CLL 2. To study PI3Ki drug class activity in idelalisib-refractory CLL 3. To investigate whether ex vivo drug sensitivity can predict in vivo treatment responses Methods: CLL cells from patients that were treatment naïve (n=7), idelalisib refractory (n=9), or on idelalisib treatment (longitudinal samples from n=6 patients) were screened against 10 PI3Ki (buparlisib, compound 7n, copanlisib, duvelisib, idelalisib, nemiralisib, pictilisib, pilaralisib, umbralisib, ZSTK474), both alone and in combination with the B-cell lymphoma 2 (Bcl-2) antagonist venetoclax. Two idelalisib-resistant lymphoma cell lines were used as reference. Treatment effects on cell signaling were profiled using phospho flow and effects on cell viability were monitored with detection of cleaved caspase-3 and using the CellTiter-Glo assay. Results: Pan-PI3Ki were most effective in inhibiting PI3K signaling both after 30 min and 24h exposure. Inhibition of cell signaling correlated with induction of apoptosis. Treatment with a PI3Ki induced cell death in 72h CellTiter-Glo assays, although to varying extent. The pan-PI3Ki were overall most potent in inhibiting CLL cell viability. Interestingly, pan-PI3Ki exhibited sustained activity both in idelalisib-resistant lymphoma cell lines and in CLL cells from idelalisib-refractory patients. Combination treatment with a PI3Ki and venetoclax resulted in synergistic induction of apoptosis. Notably, CLL cells from idelalisib-refractory patients remained sensitive to idelalisib+venetoclax combination. Ex vivo drug testing on CLL cells from a patient who presented with relapsed disease after sequential treatment with FCR, ibrutinib, idelalisib and venetoclax revealed sensitivity to PI3Ki+venetoclax treatment. The patient consequently started treatment with idelalisib+venetoclax. She obtained an initial partial response, but then suffered intolerable adverse effects and the therapy was stopped. Protein profiling of the patient’s CLL cells collected at 5 time-points before, during and after the combination regimen showed that B-cell signaling was reduced by the treatment, but again significantly increased when the therapy was stopped. Increased phosphorylation of Bruton’s tyrosine kinase (BTK) and extracellular signal-regulated kinase (ERK) at the latest time-point was mirrored by increased sensitivity to venetoclax combined either with a BTK or MEK inhibitor. Unfortunately, the patient passed away and the efficacy of these alternative therapies could not be tested. To more systematically assess the predictive value of ex vivo drug sensitivity data, we analyzed drug responses to 73 combinations on CLL cells from patients treated with idelalisib. CLL cells collected at baseline from patients who obtained a long-term response to idelalisib showed significantly higher drug sensitivity scores than CLL cells from patients who obtained a short-term response. Summary/Conclusion: Our findings indicate PI3Ki drug class activity in idelalisib-refractory CLL, and suggest that ex vivo drug sensitivity may guide precision medicine and predict treatment responses. These results warrant further testing in larger cohorts and in clinical trials.